Opto-electronic systems may implement one or more feedback loops to achieve certain operational characteristics. For example, some single-frequency lasers use a passive feedback loop to correct drifts in the laser frequency to achieve a stabilized output frequency. A typical feedback loop of this type has a photosensor to convert a fraction of the laser output into an electrical signal and a device to generate a benchmark frequency to which the laser frequency is locked. A non-zero error signal is generated by the feedback loop to control the laser if the laser frequency deviates from the benchmark frequency.
A regeneratively mode-locked laser is another example of an opto-electronic system with a passive feedback loop. In a regeneratively mode-locked laser, a mode beat signal from a laser is detected by a photodetector in the feedback loop. The signal is subsequently amplified and fed back to modulate the laser. A bandpass filter may be used to filter the signal in the feedback loop. The feedback loop is usually passive and not self-oscillating. Another aspect is that the stability of the laser is dependent on the laser cavity itself and is essentially independent of the feedback loop. In addition, the stability of the mode beating signal is almost entirely determined by the laser cavity. See, for example, Nakazawa et al., "Ultrastable Harmonically and Regeneratively Modelocked Polarisation-Maintaining Erbium Fibre Ring Laser", Electronics Letters, Vol. 30, No. 19, pp. 1603-1605, Sep. 15, 1994; Nakazawa and Yoshida, "Direct Generation of a 750fs, 10 GHz Pulse Train from A Regeneratively Mode-Locked Fibre Laser with Multiple harmonic modulation", Electronics Letters, Vol. 32, No. 14, pp. 1291-1293, Jul. 4, 1996; and Kinsel, "A Stabilized Mode-Locked Nd:YAlG Laser Using Electronic Feedback", IEEE Journal of Quantum Electronics, Vol. QE-9, No. 1, pp. 3-8, January, 1973.
In addition to passive feedback loops, an active opto-electronic feedback loop with an open-loop gain less than unity may also be used in an opto-electronic system. Such an active feedback loop not only can provide a feedback to alter the operation of the system but also can generate and sustain an electromagnetic oscillation in the loop.
An opto-electronic oscillator ("OEO") has one or multiple active feedback loops to generate both optical modulation and electrical oscillation in radio frequency spectrum. An OEO usually has an electro-optic modulator pumped by a laser and at least one active opto-electronic feedback loop which provides in-phase feedback to an RF input port of an electro-optic light modulator. The open loop gain in the loop is greater than unity in order to generate an RF electrical oscillation therein. The pump laser is usually a single-mode laser. The continuous photon energy from the laser is converted into RF or microwave signals by the feedback loop.
The feedback loop in an OEO can be electrical and/or optical in nature, allowing both signal output and signal injection in either electrical or optical format or a combination thereof. An OEO is capable of producing spectrally pure RF oscillations with excellent stability, frequency tunability and low phase noise. The high performance and adaptability for both optical and electrical domains make OEOs suitable to a variety of applications for photonic communication and data processing systems. Detailed information on opto-electronic oscillators can be found, for example, in U.S. Pat. No. 5,723,856 issued on Mar. 3, 1998 for a single-loop OEO by Yao and Maleki, and U.S. Pat. No. 5,777,778 issued on Jul. 7, 1998 for a multiple-loop OEO by Yao.